A relatively new ASHRAE standard abbreviated MERV shows a filter's minimum performance through its life, allowing the contractor or building owner to select filters knowing their “worst case” efficiency. It measures a filter's ability to remove particles of specific sizes when the filter is new. The old standard does not tell you a filter's efficiency in removing specific particle sizes (such as lung-damaging respirable particles). By comparison, with the ASHRAE 52.2-1999 test, particle counters measure the number of airborne particles with diameters of 0.3 to 10.0 microns, both upstream and downstream of the air filter. Using this information, it becomes possible to take a highly targeted approach to filter selection. Once the test is completed, the filter's minimum efficiency values at various particle sizes are recorded. These efficiency values are then used to assign a MERV number to the filter. Designations range from MERV 1 (typically a low efficiency, throwaway filter) up to MERV 16 (a 95%-plus efficiency filter). The new MERV system is much more comprehensive than previous systems, and it enables one to compare efficiencies of filters at a glance. Minimum Efficiency Reporting Value, or MERV for short, is a filter rating system devised by the American Society of Heating, Refrigeration and Air conditioning Engineers (ASHRAE) to standardize and simplify filter efficiency ratings for the public. The higher the MERV rating, the higher the efficiency of the air filter. Simply stated, a MERV 12 filter will remove more smaller particles from the air than a MERV 8 filter.
The MERV system allows the consumer to effectively compare one brand of filter to another. Without any value-added additions, any MERV 6 filter will perform about the same as any other MERV 6 filter. The MERV rating only applies to efficiency of the filter. The presence of other functional ingredients in the filter, such as anti-bacterial treatment and/or baking soda, are value-added benefits and are not a part of the MERV rating system. In the 3-10 micron range of particle diameters, a MERV 6 filter will be about 35-50 percent efficient. A particle that is 10 microns or less in size is not visible to the naked human eye. If allergies or asthma are your concern, we suggest you choose a MERV 8 filter at a minimum.
In the past furnace filters were made from relatively coarse continuous fibers laid down in a random pattern and built up in layers on a drum and by carding dry staple fibers and forming webs of the carded fibers. Such filters, in recent years at least, fall short of removing as small of particles and as many particles as desired. Average efficiency is really not a realistic measure of filter performance because it exaggerates performance for the early part of the filter's actual service life. This is because when an air filter is first installed its efficiency is at its lowest point because it hasn't built up enough lint and particles on the filter to help trap more and smaller lint and particles. Some of these filters had low efficiencies and others had other disadvantages such as low physical integrity and high bulk preventing pleating or making it very difficult. Also, cost is an issue. In U.S. Pat. No. 6,579,350 some of this is confirmed and addressed by using a layer of wet laid fibers to attempt to minimize the shortcomings of dry or air laid filtration media, but that process and filter is still complex and expensive.